Within the ever evolving telecommunications industry, the advent of numerous independent, localized networks has created a need for reliable inter-network communication. Unfortunately, this inter-network communication is difficult to accomplish in a cost effective manner due to differences in the digital signal hierarchies, the encoding techniques and the multiplexing strategies. Transporting a signal to a different network requires a complicated multiplexing/demultiplexing, coding/decoding process to convert the signal from one scheme to another scheme. A solution to this problem is SONET, an acronym for Synchronous Optical NETwork. It is an optical transmission interface, specifically a set of standards defining the rates and formats for optical networks. Proposed by Bellcore during the early 80s and standardized by ANSI, SONET is compatible with Synchronous Digital Hierarchy (SDH), a similar standard established in Europe by ITU-T. SONET offers a new system hierarchy for multiplexing over modern high-capacity fiber optic networks and a new approach to Time Division Multiplexing (TDM) for small traffic payloads. SONET has several advantages, including:
meeting the demands for increased network Operation and Maintenance (OAM) for vendors and users by integrating the OAM into the network, thus reducing the cost of transmission; PA1 standardizing the interconnection between different service providers (Mid-Span Meet); PA1 allowing the adding and/or dropping of signals with a single multiplexing process, as a result of SONET's synchronous characteristic. PA1 a plurality of primary receive connections for receiving inter-ring optical signals divided into second size blocks, where a second size block is smaller than a first size block, a second size block including a header section and a payload section, the header section including a mark related to at least a portion of the payload section, the mark being indicative as to whether the portion of the payload section is concatenated data or non concatenated data, when the portion of the payload section is non concatenated data it includes a plurality of independent data elements; PA1 a plurality of primary send connections for transmitting inter-ring optical signals divided into second size blocks; PA1 a secondary receive connection for receiving intra-ring optical signals divided into first size blocks; PA1 a secondary send connection for transmitting intra-ring PA1 optical signals divided into first size blocks; PA1 a routing controller for routing of first size blocks towards the primary send connections, said routing controller being operative to: PA1 a plurality of primary receive connections for receiving inter-ring optical signals divided into second size blocks, where a second size block is smaller than a first size block, a second size block including a header section and a payload section, the header section including a mark related to at least a portion of the payload section, the mark being indicative as to whether the portion of the payload section is concatenated data or non concatenated data, when the portion of the payload section is non concatenated data it includes a plurality of independent data elements; PA1 a plurality of primary send connections for transmitting inter-ring optical signals divided into second size blocks; PA1 a secondary receive connection for receiving intra-ring optical signals divided into first size blocks; PA1 a secondary send connection for transmitting intra-ring optical signals divided into first size blocks; PA1 said method comprising the steps of:
The Synchronous Transport Signal (STS) frame is the basic building block of SONET optical interfaces, where STS-1 (level 1) is the basic signal rate of SONET. Multiple STS-1 frames may be concatenated to form STS-N frames, where the individual STS-1 signals are byte interleaved. The STS frame comprises two parts, the STS payload and the STS overhead. The STS payload carries the information portion of the signal, while the STS overhead carries the signaling and protocol information. This allows communication between intelligent nodes within the network, permitting administration, surveillance, provisioning and control of the network from a central location. At the ends of a communication system, signals with various rates and different formats must be dealt with. A SONET end-to-end connection includes terminating equipment at both ends, responsible for converting a signal from the user format to the STS format prior to transmission through the various SONET networks, and for converting the signal from STS format back to the user format once transmission is complete.
SONET is a four layer system hierarchy, with each layer building on the services provided by the lower layers. Each layer communicates to peer equipment in the same layer, processes information and passes it up and down to the next layer. The path layer ensures the end-to-end transport of user data at the appropriate signaling speed, mapping services (such as DS1, DS2, DS3 and video) and path overhead into Synchronous Payload Envelopes (SPEs). The line layer multiplexes and synchronizes the SPEs and adds line overhead to form STS-N combined signals. The section layer performs scrambling and framing, and adds section overhead, in order to create SONET frames. Finally, the photonic layer is the SONET physical layer, converting electrical signals into optical ones and transmitting these to distant nodes. At the distant nodes, the process is reversed, starting with the photonic layer, whereby the optical signal is converted to an electrical signal and passed down to the path layer where the different service signals terminate. The optical form of the STS signals are called Optical Carriers (OCs). The STS-1 signal and the OC-1 signal have the same rate.
The SONET line rate is a synchronous hierarchy that is flexible enough to support many different capacity signals. The STS-1/OC-1 line rate was chosen to be 51.84 Mbps to accommodate 28 DS1 signals and 1 DS3 signal. The higher level signals are obtained by synchronous multiplexing of the lower level signals. This higher level signal can be represented by STS-N or OC-N, where N is an integer. Currently the values of N are 1, 3, 12, 48 and 192. For example, OC-48 has a rate of 2488.320 Mbps, 48 times the rate of OC-1. SONET is also capable of handling signals that are at a lower rate than the STS-1 signal, referred to as sub-STS-1 signals. Specific to SONET, the sub-STS-1 signal is referred to as a Virtual Tributary (VT), where the VT structure is designed for transporting and switching sub-STS-1 signals. There are four types (sizes) of VTs, specifically VT-6, VT-3, VT-2 and VT-1.5, each size designed to accommodate a certain size of digital signal, respectively DS2 (6.312 Mbps), DS1C (3.152 Mbps), CEPT-1 (2.048 Mbps) and DS1 (1.544 Mbps).
Since an optical cable is capable of transmitting very high data rates, it is logical to multiplex multiple STS-1 signals to fully utilize the network capacity. Such multiplexing is required to provide super rate services such as BISDN. In the SONET synchronous environment, multiple STS-1 signals are traveling together at a higher rate, but are still visible as individual STS-1 signals as a result of the interleaving process. For the purposes of this specification, interleaving is a procedure for interlacing the individual bytes of a signal such that each component signal is visible within the combined signal. This eliminates the necessity for complete demultiplexing of an STS-N signal in order to access a single STS-1 signal.
Existing optical networks are formed by several inter-connected rings, each ring formed itself by several nodes connected to one another. Two rings may be simply inter-connected via a line connection between one node from each ring. Unfortunately, this inter-ring connection may fail as a result of either a node failure, a line failure, a path failure or a channel failure, all situations resulting in a loss of traffic within the network. A channel failure consists in a failure of one of the multiple signal channels, for instance one of the 12 STS-1 channels forming an OC-12c. A path failure consists in a failure of one of the two paths forming a send-receive connection between two transmission nodes within the optical network. A line failure consists in a failure of the fiber optic line, and thus of the entire signal.
A solution to this inter-ring connection failure problem is to provide protection switching, whereby a secondary connection between rings establishes an alternate route of transmission. Within this network structure, nodes establishing a primary connection between rings have the capability to route data over a secondary connection in the case of a primary node, line or channel failure. This network solution, commonly referred to as Matched Nodes, is currently in use within optical networks and is compliant to Bellcore's GR1230 requirements. The Matched Nodes approach provides a mechanism to protect one STS-1 channel, as the inter-ring switching only takes place at this facility level, and can be used between any two types of rings, such as an OC-12, OC-48 or OC-192 ring, among others.
Unfortunately, the Matched Nodes approach is limited to protecting a single STS-1 channel at a time. Progressively, more and more optical network customers are asking for the transmission of concatenated payloads, which conflicts with the Matched Nodes requirements. Another limitation of the Matched Nodes approach is the time required to execute a protection switch between inter-connected rings. A 100 msec hold-off time is applied before every protection switch, whether an intra-ring path, inter-ring line or inter-ring path failure has occurred. Also, in the case of large payload transmission whereby only one STS-1 signal can be switched at a time, the processing time of a protection switch between rings may become larger than the time required as per the Bellcore specifications. For example, in the case of an OC-48c optical signal, 48 OC-1 service selectors would have to switch.
The background information provided above clearly indicates that there exists a need in the industry to provide a method and apparatus for concatenated payload transmission in synchronous optical networks.